We are running a 3.1.2.0 ADSM server under MVS. Incoming backups are written
directly to a Virtual Tape Subsystem. The VTS emulates a large number of 3490
tape drives, buffers data on disk, and eventually stacks 3490 volume images on
real 3590 volumes. From the standpoint of megabytes transferred over the
network and megabytes stored on tape, our workload consists mainly of database
files that change every day. The rest of the workload consists of an
assortment of files that change with widely varying frequencies. We have set
up management classes that send the database backups to a separate tape
storage pool. The goal of this arrangement is to have most of our tapes become
empty as their contents expire, with no need for reclamation. When ADSM does
an incremental backup of a particular client system, it alternates between
sending a group of one or more database files and sending a group of one or
more non-database files. It appears to dismount the current tape and mount a
different tape at each transition between the two classes of files. This
happens even though the mount retention period and the mount limit would allow
for keeping one database tape and one non-database tape mounted at any given
time throughout the backup, switching the flow of data between them as needed.
The time consumed by the tape mounts does not appear to be a major issue. The
disk buffering in the VTS allows the mount requests to be satisfied in about
two seconds each. The total amount of time spent on the mounts is small
compared to the overall duration of the backup. What concerns me is the impact
on the usage of real 3590 volumes. As I understand matters, the VTS writes the
current contents of a virtual 3490 to a real 3590 each time the virtual volume
is dismounted. Repeated dismounts of partially filled virtual volumes will
waste large amounts of space on the real 3590 volumes and eventually trigger
large amounts of VTS reclamation activity (the VTS has its own tape
reclamation process, analogous to the one performed by ADSM). Is there any way
to eliminate the unnecessary mounts and dismounts?